Continuous production of hollow ingots
The present invention relates to a continuous method for the production of hollow quartz-glass ingots and an apparatus for said method, whereby the hollow quartz-glass ingots are cooled on the internal surface.
1. A method for producing a hollow quartz-glass ingot, characterized by the following process steps:
a. providing a softened quartz-glass mass in a crucible or refractory tank;
b. vertically drawing off the softened quartz-glass mass through a die in which is mounted a mandrel, to provide a hollow quartz-glass ingot; and
c. on-line cutting of the hollow quartz-glass ingot to a specific length,
characterized in that the internal surface of the hollow quartz-glass ingot is cooled by active means after vertically drawing off the softened quartz-glass mass in step b. and before on-line cutting in process step c.
2. The method according to claim 1 , characterized in that step b. is carried out with a mandrel mounted on a mandrel support column, and the internal surface of the hollow quartz-glass ingot is cooled by one or more of the following means
1) by a coil arranged around the mandrel support column, whereby said coil comprises a cooling fluid;
2) by the mandrel support column itself which is cooled with a cooling fluid arranged inside the mandrel support column;
3) by a gas flow of an inert gas in the inner cavity of the hollow quartz-glass ingot; and/or
4) by a water spray issuing as multiple jets or as an atomized mist of fine droplets.
3. A method according to claim 1 , wherein the mandrel is supported from below on a mandrel support column, and means are provided to hold the mandrel in place temporarily while said column is detached from the bottom carriage and each cut ingot section is removed, and said column is subsequently reattached to the bottom carriage.
4. The method according to claim 1 , characterized in that the external surface of the hollow quartz-glass ingot is cooled by enhancing active means of a cooling gas, a cooling fluid, active convection and/or radiation.
5. The method according to claim 1 , characterized in that the hollow quartz-glass ingot is cooled to a temperature of the internal surface of the hollow quartz-glass ingot of less than 400° C. prior to prior to the on-line cutting in process step c.
6. The method according to claim 1 , characterized in that the hollow quartz-glass ingot is cooled to a temperature of the external surface of the hollow quartz-glass ingot of less than 400° C. prior to cutting in process step c.
7. The method according to claim 1 , characterized in that the temperature difference between the internal surface and the external surface of the hollow quartz-glass ingot at the cutting zone is less than 300° C.
8. The method according to claim 1 , characterized in that the total stress of the hollow quartz-glass ingot prior to the cutting in step c. is reduced to less than 5 MPa.
9. The method according to claim 1 , characterized in that the process step c. is carried out in a cutting station being spaced 1800 mm to 4000 mm from the die orifice along the hollow quartz-glass ingot extruded from the die orifice.